The explosive growth of both the wireless industry and the Internet is creating a huge market opportunity for wireless data access. Limited internet access, at very low speeds, is already available as an enhancement to some existing cellular systems. However those systems were designed with purpose of providing voice services and at most short messaging, but not fast data transfer. Traditional wireless technologies are not very well suited to meet the demanding requirements of providing very high data rates with the ubiquity, mobility and portability characteristics of cellular systems. Increased use of antenna arrays appears to be the only means of enabling the type of data rates and capacities needed for wireless internet and multimedia services. While the deployment of base station arrays is becoming universal it is really the simultaneous deployment of base station and terminal arrays that can unleash unprecedented levels of performance by opening up multiple spatial signaling dimensions .Theoretically, user data rates as high as 2 Mb/sec will be supported in certain environments, although recent studies have shown that approaching those might only be feasible under extremely favorable conditions-in the vicinity of the base station and with no other users competing for band width. Some fundamental barriers related to the nature of radio channel as well as to the limited band width availability at the frequencies of interest stand in the way of high data rates and low cost associated with wide access.
FUNDAMENTAL LIMITATIONS IN WIRELESS DATA ACESS
Ever since the dawn of information age, capacity has been the principal metric used to asses the value of a communication system. Since the existing cellular system were devised almost exclusively for telephony, user data rates low .Infact the user data were reduced to the minimum level and traded for additional users. The value of a system is no longer defined only by how many users it can support, but also by its ability to provide high peak rates to individual users. Thus in the age of wireless data, user data rates surges as an important metric.
Trying to increase the data rates by simply transmitting more; Power is extremely costly. Furthermore it is futile in the contest of wherein an increase in everybody’s transmit power scales up both the desired signals as well as their mutual interference yielding no net benefit.
Increasing signal bandwidth along with the power is a more effective way of augmenting the data rate. However radio spectrum is a scarce and very expensive resource.Moreover increasing the signal bandwidth beyond the coherent bandwidth of the wireless channel results in frequency selectively. Although well-established technique such as equalization and OFDM can address this issue, their complexity grows with the signal bandwidth. Spectral efficiency defined as the capacity per unit bandwidth has become another key metric by which wireless systems are measured. In the contest of FDMA and TDMA, the evolutionary path has led to advanced forms of dynamic channel assessment that enable adaptive and more aggressive frequency reuse.In the context of multi-user detection and interference cancellation techniques.